This invention relates to the signal transmission technique. Particularly, this invention relates to the method and system for transmitting and receiving quadrature-amplitude modulation signals (QAM) with the low synchronization threshold on the carrier frequency.
In transmitting and receiving signals modulated in one or another manner, a very important characteristic is the demodulation threshold, i.e., the ratio of the signal power to the noise power (signal-to-noise ratio, SNR), at which the carrier wave of the signal being received ceases to be derived, which results in loss of the reception. The demodulation threshold depends essentially on the demodulation type employed at the transmission side, and the noiseless coding type.
It is known from the theory that effectiveness of any communication system is defined by the frequency and power resources thereof, i.e., by the bandwidth occupied with the signal, and by the signal power for providing required rates for transmitting and receiving information. In general, this dependence of the rate for transmitting and receiving information upon the frequency spectrum width and signal power is defined by Shannon's equation:
                              C          =                      B            ⁢                                                  ⁢                          log              2                        ⁢                                                            P                  S                                +                                  P                  N                                                            P                N                                                    ,                            (        1        )            
where C is the information transmission and reception rate, B is the frequency bandwidth of the signal being transmitted in a communication channel, PC is the signal power at the receiver input, PN is the noise power reduced to the receiver input in the bandwidth B.
Modern communication systems represent modern technologies constructed for specific information transmission rates. The following modulation types are used most often:                in the satellite communication: QPSK, 8PSK, 16QAM, 32QAM;        in relay repeater lines: BPSK, QPSK, 8PSK, 16QAM, 32QAM, 64QAM, 128 QAM, 256QAM;        in cable lines: QPSK, 16QAM, 64QAM, 256QAM;        in telephony: from 16QAM to 16384QAM.        
The most often used types of noiseless coding employed in modern modems are Viterbi (convolutional) coding, coding with Reed-Solomon codes, trellis code modulation (TCM), turbocoding [1], and low density parity check (LDPC) coding [3, 4]. The latter is the most effective type of noiseless coding that allows, with the loss of only 0.8-1.5 dB, to achieve the maximum information transmission rates defined by the equation (1). The Table 1 shows the LDPC coding characteristics for various coding rates and modulation types.
The obstacle for implementing the achieved characteristics of the LDPC coding in the modern communication systems is too high demodulation thresholds (carrier recovery thresholds) in the existing demodulators. Thus, for the QPSK type demodulation, the existing demodulators begin the carrier synchronization at the S/N ratio of about 0 dB, for the 16QAM type modulation at the S/N ratio of about +8.9 dB, and for the 32QAM type modulation at the ratio of about +12.7 dB [2].
TABLE 1SpectraleffectivenessModulationS/NShannon's S/N(bps/Hz)typeCoding rate(dB)threshold (dB)0.5QPSK1/4−2.35−3.870.666QPSK1/3−1.24−2.20.8QPSK2/5−0.3−1.31QPSK1/2101.2QPSK3/52.231.11.5QPSK3/44.032.51.6QPSK4/54.683.10.758QAM1/4−0.8−1.718QAM1/30.701.28QAM2/51.8511.58QAM1/23.42.51.88QAM3/553.8528QAM2/36.24.772.258QAM3/47.55.8116QAM1/40.501.33316QAM1/32.21.61.616QAM2/53.53216QAM1/25.54.77316QAM3/410.18.453.216QAM4/5119.21.2532QAM1/41.81.351.6632QAM1/33.753.1232QAM2/55.44.772.532QAM1/27.56.7332QAM3/59.58.5432QAM4/513.511.751.564QAM1/42.752.5264QAM1/354.772.464QAM2/56.66.25364QAM1/298.453.664QAM3/511.110.45464QAM2/312.511.774.564QAM3/414.513.3
One can see from the Table 1 that for implementing the entire possibility of the LDPC coding for the QPSK signal at the coding rate of ¼, the demodulator should operate at the ratio
            S      ⁢              /            ⁢      N        =                  10        ⁢                  log          2                ⁢                              P            S                                P            N                              =                        -          2.35                ⁢                                  ⁢        dB              ,while it loses the synchronization as early as 0 dB. For the 16QAM signal at the same coding rate of ¼, the demodulator should have the stable operation at the ratio
            S      ⁢              /            ⁢      N        =                  10        ⁢                                  ⁢                  log          2                ⁢                              P            s                                P            N                              =                        -          0.5                ⁢                                  ⁢        dB              ,while it loses the synchronization as early as +8.9 dB, and so on.
The main reason of this appears from the fact that the system for carrier recovery in the modern QPSK and QAM demodulators is non-linear. There is no carrier residue in the spectrum of signals using such modulation types as QPSK, 8PSK, 16QAM, etc., therefore the wave coherent to the carrier is derived from the signal being received by means of some non-linear transformation and following filtration. But any non-linearity restricts the carrier recovery threshold. If only the carrier recovery system is linear, then the demodulation threshold would be less than −3 dB, which would permit the demodulator to keep its characteristics up to the ratio
      S    ⁢          /        ⁢    N    =            10      ⁢                          ⁢              log        2            ⁢                        P          s                          P          N                      =                  (                                            -              6                        ⁢                                                  ⁢            to                    ⁢                                          -          10                )            ⁢                          ⁢              dB        .            
So, the presently known noiseless coding systems, e.g., the LDPC coding and turbocoding, permit to come rather closely to the Shannon's threshold. However, its achievement is restrained by the absence of demodulators capable to operate at such low S/N ratios due to the absence of the synchronization, which requires to derive the carrier from signals utilizing such modulation types as QPSK, 8PSK, 16QAM, etc. using a non-linear transformation followed by filtration. The technique for frequency multiplying is such transformation, which technique can be implemented by raising the input signal to the M-th power (to the fourth power for the QPSK, to the eighth power for the PSK, etc.). But in doing so, a noise is raised to the same power. Moreover, a phase ambiguity emerges, too, which deletion requires for adding to the signal being transmitted a relative coding that introduces additional power loss.
Complexity associated with the use of the PSK, QPSK and 8PSK modulation types is obviously demonstrated in the U.S. Pat. No. 6,697,440 (published Feb. 24, 2004) and Japan Laid-out Patent Application No. 2000-032072 (published Jan. 28, 2000).
As noted preciously, the quadrature-amplitude modulation (QAM) is used amongst other modulation technique in the modern communication systems.
Thus, the Japan Laid-out Patent Application No. 2001-237908 (published Aug. 31, 2001) discloses the system for deriving the QAM synchronization signal, which system providing the quasi-synchronous detection. The U.S. Pat. Nos. 6,717,462 (published Apr. 6, 2004) and 6,727,772 (published Apr. 27, 2004) disclose the methods and systems for transmitting and receiving QAM signals with carrier adjustment. However, these both patents provide only the ordinary processing of the QAM signal. The disadvantage of these analogues is the impossibility for lowering the demodulation threshold in order to come near the Shannon's threshold.